Fixing memory leak in PreferenceScreen.

Every time the PreferenceScreen is displayed a new ListView is
created and bound to the adapter. As the same adapter is used during
the lifetime of PreferenceScreen and the listviews never gets
unbound, the adapter will contain a list of unused views. The old view
should be unbound from adapter when we create a new view.

Change-Id: I13e2d0dc79c8ff79b58efa650653e3f84c6e53c5